The present invention relates generally to patient ventilation systems and, more particularly, to a bi-directional flow sensor having improved accuracy in measuring respiratory flow to and from a patient.
Mechanical ventilators are used to provide respiratory support to a patient by assisting in the inhalation and exhalation phases of the breathing cycle. In one arrangement, the mechanical ventilator may be connected to the patient by a wye fitting. The wye fitting is, in turn, fluidly connected to the patient's airway by a patient tube connected to a patient interface. The wye fitting may have an exhalation valve connected to one leg of the wye fitting.
The exhalation valve is moved between open and closed positions according to the phase of the breathing cycle. During the inspiration phase, the exhalation valve is closed to allow compressed gas from the ventilator to be delivered to the patient. During the exhalation phase, the exhalation valve opens to allow the patient to exhale to atmosphere. In certain ventilator arrangements, a positive end expiratory pressure (PEEP) valve is used in combination with the exhalation valve in order to provide an elevated back-pressure above atmosphere during the exhalation phase.
A flow sensor is used to determine the flow rate of compressed gas passing from the ventilator to the patient as well as determine the flow rate of exhalation gas flowing from the patient to the exhalation valve. Differential pressure detection is one of the more common techniques for measuring flow of a gas. Differential pressure flow sensors include a flow restrictor positioned within the flow of gas passing through the sensor to allow measurement of the pressure drop (i.e., the differential pressure) that occurs across the flow restrictor. Bi-directional flow sensors are capable of determining flow rate in either direction as a function of the measurable pressure difference between upstream and downstream pressure taps on opposite ends of the flow restrictor. The measurable pressure difference is correlated to an empirically-established flow rate.
In some cases, the patient interface is provided as an endotracheal tube for delivering pressurized gas from the mechanical ventilator to the patient. The endotracheal tube is typically of a relatively small diameter. An airway adapter is used to mate the small diameter endotracheal tube to the larger diameter flow sensor fitting which is available in standard sizes. The flow sensor is preferably located as close to the patient as possible and, in some prior art arrangements, the flow sensor may be incorporated into the wye fitting or may be located between the wye fitting and the patient interface.
Because of the size discrepancy between the relatively small diameter endotracheal tube and the larger diameter flow sensor, exhalation by the patient results in a relatively high velocity pressure jet exiting the endotracheal tube and entering the flow sensor. The artificially high velocity pressure from the endotracheal tube impinges on the pressure taps of the flow restrictor in the flow sensor. This high velocity pressure jet results in an artificially high differential pressure measurement for the given flow relative to the empirically-established flow rate/differential pressure relationship. The result is an artificially high flow rate measurement.
In an attempt to overcome the problem of an artificially high flow velocity generated by the pressure jet, some prior art ventilation systems increase the distance from the endotracheal tube to the flow sensor by approximately six inches. This increased distance between the flow sensor and the endotracheal tube permits the pressure jet to more uniformly disperse within the flow sensor prior to impinging upon the pressure taps. In this manner, the flow velocity is relatively constant across the cross-sectional area of the flow sensor such that pressure measurements are believed to be more accurate. Unfortunately, the increase in distance from the flow sensor to the endotracheal tube also increases the amount of re-breathed volume or deadspace in the patient's airway. The increased deadspace results in re-breathing of previously exhaled gasses.
Another problem associated with flow measurement is that during the inhalation phase, inaccurate pressure measurements at the flow sensor can occur as a result of pneumatic noise in the flow. Such pneumatic noise may include turbulence, vibrations, or asymmetric flow conditions at the ventilator end of the flow sensor (i.e., opposite the patient end). Certain mechanical ventilation systems are configured to operate with a bias flow which may include pneumatic noise. For example, the mechanical ventilator system similar to that disclosed in U.S. Pat. No. 6,102,038 issued to DeVries operates with a bias flow which circulates through the wye fitting depending on whether the exhalation valve is open or closed.
For most applications, the bias flow is typically in the range of about 2-10 liters per minute (LPM) and can introduce pneumatic noise at the flow sensor which reduces the accuracy of the flow sensor. The pneumatic noise in the bias flow may be the product of asymmetric flow conditions at the inlet to the flow sensor. More specifically, because of the geometry of the wye fitting, the bias flow may enter the flow sensor in a non-axial direction creating a flow vortex or cross flow at the flow sensor which results in inaccurate pressure measurement at the pressure taps of the flow sensor.
Pressure sensed in the flow sensor can be used to cycle the mechanical ventilator exhalation valve according to patient-initiated inspiration and exhalation phases of each breathing cycle. Particularly for neonatal and pediatric patients, it is desirable to minimize pneumatic noise in the bias flow such that the 0.2 LPM flow rate at which the inspiration and exhalation phases are triggered, is not disturbed by the pneumatic noise. In this regard, it is desirable that such pneumatic noise is maintained at or below 0.1 LPM.
As can be seen, there exists a need in the art for a flow sensor that is adapted for use with neonatal and pediatric patients. More specifically, there exists a need in the art for a flow sensor that can operate with reduced pneumatic noise such that patient-initiated inspiration and exhalation phases of each breathing cycle are triggered at the appropriate flow rate. Additionally, there exists a need in the art for a flow sensor that is adaptable for use with small diameter endotracheal tubes.
Preferably, the flow sensor is configured to eliminate the artificially-high pressure measurement produced by the pressure jet discharged from the endotracheal tubes during exhalation. Furthermore, it is desirable that the flow sensor is configured to minimize deadspace in order to prevent CO2 re-breathing by the patient. Finally, there exists a need in the art for a flow sensor which overcomes the adverse effects of pneumatic noise at the ventilator end while minimizing resistance to airflow during inspiration and exhalation.